As noted in U.S. Pat. No. 2,744,043, entitled "Method of Producing Pressure Containers for Fluids", glass fiber threads or threads of other suitable material are applied in a mutually superimposed relationship on a removable core member, in a method generally known as the filament winding process, to manufacture pressure containers.
The threads are impregnated in situ, as the application of successive layers of threads proceeds, with a thermosetting plastic resinous bonding medium. The bonding medium fills the interstices between the threads, and the interstices between the layers of threads, so as to form, after curing, a substantially rigid structure having an inner surface and an outer surface.
During the initial filament winding process additional mechanical stiffeners having a simple surfaces formed parallel to the inner surface may be added, for example at the neck of the container to define a threaded attachment. Such a process yields a structure having a substantially planar interior and exterior surface, with no opportunity for other mechanical stiffening inserts. Areas of localized high stresses must be reinforced with additional layers of threads, requiring the overwinding of areas that do not necessarily require additional reinforcement. The resultant composite structure therefore tends to be overweight and therefore undesirable in, for example, an aerospace application.
The core member may be removed piece by piece from the interior of the cured structure such as, for example, as disclosed in U.S. Pat. No. 3,220,910, by sawing and removing portions of a hard plaster mandrel from the interior of the cured structure. Alternatively, the core member may comprise an inflatable mandrel positioned within the windings which may be deflated for removal.
The use of an inflatable mandrel for the core member causes problems in the shape of the final part because the mandrel tends to sag or deform during the winding process. As described in U.S. Pat. No. 4,123,307, this problem may be solved by the spray-deposition of a hard plastic material on the outer surface of the inflatable mandrel while it is rotating in a fixture, so as to form a self-supporting hard shell about the mandrel. Glass fibers or other filamentary material are then wound against the hard shell as it is rotated in a filament winding fixture. The shape of the hard shell attempts to accurately define the interior surface of the cured composite structure.
It can therefore be seen that it is desired to have the foundation of a hard surface upon which to build a composite structure. The interior surface of the uncured portions of a composite structure needs to be placed against a hard structure, so as to eliminate distortion or sagging in the structure, prior to the curing process.
In a similar manner it is also necessary to insure the dimensional accuracy of the exterior surface of the composite structure.
It would seem that the optimum composite formation process would merely require the positioning of the uncured portions of the composite structure against a mandrel with a hard interior surface, followed by surrounding the uncured portions with a hard exterior shell having the required exterior dimensions, and thereafter applying sufficient heat to cure the structure.
Unfortunately, during the curing process composite structures tend to "debulk", or shrink up to 20 to 25%, such that the originally uncured (as wound) wall thickness decreases substantially. With the composite structure initially positioned between two hard surfaces, during the curing process the composite structure would tend to pull away from both surfaces, and therefore yield a part having questionable surface dimensionsal accuracy, as well as poor laminate compaction.
The inflatable mandrel concept attempts to solve the debulking problem by applying pressure to the interior of the part during the curing cycle, so as to maintain pressure against the interior of the composite structure and thereby force and compact the structure up against the hard exterior surface. But, as mentioned earlier, the initial assembly of the composite structure about the dimensionally unstable inflatable mandrel cannot guarantee accurate dimensional control of the composite's interior surface. The cylindrical shape of the inflatable bladder also does not, for example allow for any integrally co-cured beam-shaped stiffening inserts to be included about the interior surface of the structure.
A method therefore needs to be developed that allows the fabrication of a composite structure having accurately dimensioned interior and exterior surfaces. The method should also allow for the inclusion of stiffening members wherever required in the composite structure, the stiffening members forming ribs or blades located circumferentially and/or longitudinally as required about either the interior or the exterior of the structure. In fact most structures require both stiffeners, creating a crossing beam structure of ring frames and longerons.